There currently exists a number of methods for the selective separation of gaseous feed stream components, including the removal of light olefins from gas streams containing other non-olefinic components. While there has been interest in using membranes as a means of separation, the lack of high flux and high selectivity membranes economically operable in the industrial setting has hindered the application of membranes for this purpose. Ideal characteristics for membranes for separation purposes include the ability of the membrane to maintain its integrity under high pressure and severe environmental conditions; to achieve high performance levels; to maintain high performance levels for an economical period of time; to produce consistent and reliable results; to be easily assembled from commercially available components or to be manufactured with relative ease; and to be technically feasible. The present state of membrane technology offers a number of different membrane systems which attempt to achieve the various characteristics stated above.
Cryogenic distillation is the primary means used commercially to separate feed streams which are gaseous under ambient conditions. This type of process is, however, very costly, both in terms of capital and operating expenses, particularly for components with similar boiling ranges, such as ethylene/ethane, propylene/propane, butylenes/butanes, nitrogen/oxygen and the like.
Facilitated transport membrane technology is a known separation technique. It has been demonstrated in the laboratory for the selective separation of gas stream components, such as the removal of olefins from hydrocarbon-containing feed streams, and O.sub.2 from air. The facilitator used normally contains a metal ion with an affinity for the component to be separated. Silver is known to be especially useful for the removal of olefins, iron and cobalt complexes for O.sub.2, and organic amines for CO.sub.2, to exemplify a few such facilitators. Facilitated transport separations are normally run at relatively low pressure, or with the pressure on either side of the membrane being substantially equalized to avoid pressure differential from pushing the facilitator liquid out of the pores of the membrane support, thus forming a non-selective path of permeation (i.e. a leak). In these cases, a sweep stream is frequently employed to dilute product side component concentrations, and, hence, to increase the partial pressure driving force from the feed side to the permeate side of the membrane.
The most common form of membranes used in facilitated transport separation systems are known as supported or immobilized liquid membranes. The pores of the microporous membrane supports are charged with a solution of complexing ions with an affinity for the component to be separated. The membrane may be an immobilized liquid membrane, such as that disclosed by S. Kimura et al. in Separation Science and Technology, Vol. 15 (1980), pg. 1115-1133, wherein a flat porous cellulose membrane charged with potassium carbonate and cesium carbonate was used to selectively separate CO.sub.2 from biogas. Examples of this type of membrane typically show short membrane life due to drying of the membrane or migration of the liquid out of the membrane pores.
U.S. Pat. No. 4,750,918 discloses another type of facilitated transport involving the use of hollow fiber membranes, as opposed to flat sheet membranes. In this disclosure, the feed and recovery hollow fibers are immersed in a liquid bath to avoid drying problems often encountered with flat sheet immobilized membranes. The gases permeate through the wall of the feed fiber, diffuse across the liquid bath and permeate into the bore of the recovery fiber. This design, at the expense of permeation rate, offers a potentially longer membrane life than a flat membrane design as cited above, however it also will eventually require regeneration through system shutdown, and is restricted to low transmembrane pressure differentials.
U.S. Pat. Nos. 3,758,603; 3,758,605; 3,770,842; 3,800,506; 3,844,735 and 3,864,418 disclose extensive work in the area of membrane systems used to separate hydrocarbon feeds. These patents recite methods for the separation of aliphatically-unsaturated hydrocarbons and carbon monoxide from feed streams containing these components. The methods disclosed include liquid barrier permeation and metal-complexing techniques. The principle of operation in these techniques involves the use of a metal ion-containing aqueous liquid barrier solution which complexes with the material to be separated. Although similar to those designs cited above, here the complexed material is transferred across the barrier, due to a differential in partial pressure between the feed and product side of the barrier, and is then released on the product side of the membrane for collection. A sweep fluid, usually hexane, nitrogen or helium, is employed for the two fold purpose of: 1) diluting the product stream, thus increasing the partial pressure difference, and 2) equalizing the pressure across the membrane to avoid exceeding the bubble point and blowing the liquid out of the pores of the membrane, or to avoid membrane bursting or collapse.
In "Recent Developments in Separation Science," Vol. 9, 1986, pg. 173, Hughes, Mahoney and Steigelman reported the use of cellulose acetate hollow fiber membranes as liquid membrane supports for silver solutions for the facilitated transport of olefins. These membranes were asymmetric and thin skinned, with a dense, non-porous skin layer, i.e. a reverse osmosistype of membrane, resulting in relatively low permeation rates.
Permeability and stability problems in immobilized liquid membranes were researched by Teramoto et al., Journal of Membrane Science, 35 (1989), pg. 115-136, "Separation of Ethylene From Ethane by a Flowing Liquid Membrane Using Silver Nitrate as a Carrier." A separation system is disclosed whereby a module consisting of flowing carrier agent between two microporous membranes is used to separate ethylene from ethane in a feed stream by complexation with the carrier, and then removal by a sweep gas operated at a pressure equal to that of the feed. This is similar to the technique described by Sirkar et al. in U.S. Pat. No. 4,750,918 including the use of two membranes rather than one. A similar design was earlier discussed by Zhang Qi and E. L. Cussler in "Microporous Hollow Fibers For Gas Absorption," Journal of Membrane Science, 23 (1985), pg. 321-332.
In both of the foregoing references, a sweep gas was necessary to reduce the partial pressure of the permeate gas on the downstream side of the membrane in order to obtain permeation with low feed partial pressures and to minimize transmembrane pressures.
It is an object of the present invention to provide a membrane separation process which may operate at high pressure differential across the membrane without the need for a product side sweep, thus generating a pure product which does not require further separation.
It is a further object of the invention to provide a membrane system for the continuous separation of at least one component of a gaseous feed stream wherein a single membrane is continuously recharged by the circulation of the facilitator.